362 PHYSICS. 



of these media to an extent depending upon their indices of refraction. 

 The principle of the experimental method is simple : If a ray of liglit 

 coming from a direction parallel with the earth's motion in space be made 

 to interfere with a second ray coming from a direction 90° from this, 

 the former ray will have traveled 0.04 of a wave-length farther or less 

 far than the latter, according as the direction of its motion coincides with 

 or is opposed to the motion of the earth. Now, upon rotating the two 

 rays 90° in their own plane, the second one will now have a longer path 

 by 0.04 wavelength, making a total change in the position of the inter- 

 ference bands of 0.08 wave-length, a quality easily measurable. The 

 apparatus used is described and illustrated in the memoir. The results 

 go to show that there is no displacement of the interference bands, thus 

 contradicting the hypothesis of a stationary ether, a'hd disproving the 

 explanation of aberration hitherto generally accepted. [Am. Jour. Sci., 

 August, 1881, III, xxii, 120.) 



J. J. Thomson has given an ingenious explanation of the green phos- 

 phorescence observed in Crooke's tubes. It appears on the inner surfaces 

 of the exhausted glass tubes whenever they are exposed to the so-called 

 molecular bombardment of particles projected from the negative elec- 

 trode. Thomson points out, first, that as predicted by Maxwell, and 

 verified by Eowland, a moving electrified particle acts as a current of 

 electricity and possesses an (electro-magnetic) vector-potential. Now, 

 where such an electrified particle strikes a glass surface and rebounds, 

 its change of velocity is accompauie<l by a change of vector-potential, 

 and the glass against which it impinges and rebounds will be subjected 

 to rapid changes in electromotive force. But by Maxwell's theory of 

 light this is precisely what happens when a ray of light falls on it; and, 

 therefore, it phosphoresces as it would under the impact of an actual ray 

 of light. {Nature May 19, 1881, xxiv, 66,.) 



Treve has shown the curious fact that apparently, when light from 

 a natural or artificial source is admitted through a slit, more light 

 passes when the slit is horizontal than when it is vertical. Photographs 

 were taken behind slits in various positions to prove that the phenom- 

 enon is not an illusion of the eye. {Nature, April, 1881, xxiii, 616.) 



Young and Forbes have employed Fizeau's toothed-wheel method to 

 determine the velocity of light. Instead of a.single reflector at a dis- 

 tance, two were used, one a quarter of a mile behind the other. Two 

 rays were also used, which were observed when equally bright, a point 

 reached by adjusting the speeds of the toothed wheels. The general 

 result reached was that the velocity of the light of an electric lamp is 

 187,273 miles per second in vacuo. Noticing one day that one of the 

 stars looked reddish, the other bluish, the former increasing in intensity 

 with the speed of the wheel, the latter decreasing, the authors concluded 

 that the blue rays must move faster than the red ones, and instituted 

 direct experiments to test the question. As a mean of 37 determina- 



